1. Field
The disclosed subject matter relates to an LED drive circuit, and particularly to an LED drive circuit capable for a light emitting diode (LED) that maintains a constant brightness even if temperature environments such as ambient temperature and self-exothermicity vary.
2. Description of the Related Art
Performance of a PN-junction bandgap in an LED has temperature dependence and can decrease as the temperature rises. Therefore, the forward voltage of the LED has negative temperature dependence and lowers characteristically as the temperature rises.
When the LED is constant-voltage driven, therefore, an elevation in the temperature at the LED due to the ambient temperature and self-exothermicity can lower the forward voltage and increase the forward current of the LED.
The increased forward current promotes further elevation of the temperature and the forward voltage proceeds lowering. This is a state of positive feedback in terms of power, which can eventually lead to thermal runaway, which is a concern.
To avoid such issues, a method has been proposed in which a constant-current drive is provided for the LED, as shown in the circuit diagram of FIG. 4. In this case, an LED unit LU of circuitry including serially connected n LED1-LEDn is connected serially to a current sense resistor RS, and the other end of the current detection resistor RS is grounded.
An LED drive current IL (current value Il) flowing in the LED unit LU causes a voltage VRS (voltage value Vrs)=Il×Rs across the current sense resistor RS (resistance Rs), which is fed back to a feedback terminal FB of an output control circuit OCC.
A reference voltage Vref is set to the output control circuit OCC, and a difference between the voltage value Vrs of the feedback voltage VRS fed to the feedback terminal FB and the reference voltage Vref is used to control a transistor Q for controlling the output voltage Vo.
This makes it possible to control the LED drive current IL flowing in the LED unit LU connected in parallel with a capacitor C. This feedback control is continued until the voltage value Vrs of the feedback voltage VRS reaches the same potential as the reference voltage Vref. Thus, the LED drive current IL flowing in the LED unit LU can be held constant.
Such feedback technology which keeps constant the value Vrs of the voltage across the current sense resistor RS serially that is connected to the LED unit LU is a current manner in which a step-up DC/DC converter of the so-called constant-current type is configured (see Japanese Patent Application JP 2001-215913A, for example, which is incorporated herein by reference).
From the viewpoint of circuit characteristic, the circuit for constant-current driving the LED unit LU can cause an abnormal elevation in the output voltage Vo (i.e., can cause an overvoltage) at no load (for example, when a malfunction occurs in at least one of the LEDs contained in the LED unit LU resulting in a short across the malfunctioning LED(s) in the circuit). Therefore, a protective circuit can be used to counter this issue. To the contrary, even if the temperature elevation lowers the forward voltage of the LED, the drive circuit comes to a state of negative feedback in terms of power to keep the current IL flowing in the LED constant. Therefore, it advantageously operates to suppress an overcurrent in the LED, preventing the temperature in the LED from elevating due to self-exothermicity, and preventing thermal runaway from occurring.
In general, the output voltage Vo is around 30-50 V though it fluctuates in response to the fluctuation of the forward voltage of the LED because of the constant-current control as described above. The current value Il of the LED drive current IL flowing in the LED unit LU is determined on the basis of the specification required for the LED unit LU. In this case, if a higher brightness is desired, LEDs having a larger absolute maximum rating for forward current can be used in the LED unit. Therefore, the current value is determined to be substantially 0.5-1.0 A.
The above-described related art LED drive circuit is configured to exhibit a larger power loss at the current sense resistor RS and a lower efficiency of power utilization because the current sense resistor RS is connected serially to the LED unit LU.
For example, if the current value Il of the LED drive current IL flowing in the LED unit LU is 0.5 A, and the voltage value Vrs of the feedback voltage VRS placed across the current sense resistor RS is 1 V, then the current value Rs of the current sense resistor RS is given below.
  Rs  =            1      /      0.5        =          2      ⁢      Ω      
Accordingly, the power PRS consumed at the current sense resistor RS is given below.
  PRS  =                              (          IL          )                2            ×      RS        =                                        (            0.5            )                    2                ×        2            =              0.5        ⁢                                  ⁢        W            
Therefore, an increase in the current value Il of the LED drive current IL flowing in the LED unit LU in order to develop a higher brightness at the LED unit LU also increases the power PRS consumed at the current sense resistor RS, and increases the power loss correspondingly.